Studies of the identification, characterization, and chemotherapeutic regulation of lymphoid cells in man and experimental animals have demonstrated that various subpopulations of lymphoid cells not only differ markedly in their sensitivity to different chemotherapeutic regimens, but also are quite distinct both in their participation in immunologically mediated diseases, and their contribution to host defenSe mechanisms. These studies have also demonstrated that compartmentalization rather than destruction of lymphoid cells is an important mechanism of the suppression of immunological competence seen both in untreated disease states and during chemotherapeutic regimens such as corticosteroids. Corticosteroids cause a redistribution of recirculating lymphocyte subpopulations from the intravascular space to other body compartments including the bone marrow. These observations have lead to an examination and characterization of the lymphoid cell populations in the bone marrow in normal humans, in various disease states, and in experimental animals, which has resulted in clinically relevant information. The bone marrow was formerly thought to be devoid of mature thymus-derived (T) cells. Studies in our laboratory have demonstrated that the bone marrow contains a small fraction (10%) of functionally mature T cells in addition to 20 to 25% bursa equivalent (B) cells, and 70% lymphoid cells without detectable markers. These findings may well account for the frequently encountered graft-versus- host reaction seen with bone marrow transplantation as well as for the T cell functional capacity of bone marrow suspensions noted in disease states, during chemotherapeutic regimens, and in certain animal studies.